Escape velocity 2.440 km/s[f]

Rotation period synchronous[3]

Axial tilt zero[3]Greek

Albedo 0.22 (geometric)[4]

min mean max

Apparent magnitude 5.65 (opposition)[5]

Atmosphere Surfacepressure 7.5 pbar[6]

Composition ~4 × 108 cm−3 carbon dioxide[6]

up to 2 × 1010 cm−3 molecular oxygen[7]

Callisto (pronounced /kəˈlɪstoʊ/,[8] named after the Greek mythological figure

of Callisto, Greek: Καλλιστώ) is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei.[1] It isthe third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede.Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass. It is thefourth Galilean moon of Jupiter by distance, with an orbital radius of about 1,880,000 km.[2] It does notform part of the orbital resonance that affects three inner Galilean satellites—Io, Europa and Ganymede—and thus does not experience appreciable tidal heating.[9] Callistorotates synchronously with its orbitalperiod, so the same face is always turned toward Jupiter. Callisto's surface is less affected byJupiter's magnetosphere than the other inner satellites because it orbits farther away.[10]

Callisto is composed of approximately equal amounts of rock and ices, with a mean density of about1.83 g/cm3. Compounds detected spectroscopically on the surface include water ice, carbondioxide, silicates, and organic compounds. Investigation by the Galileo spacecraft revealed that Callistomay have a small silicate core and possibly a subsurface ocean of liquid water at depths greater than100 km.[11][12]

The surface of Callisto is heavily cratered and extremely old. It does not show any signaturesof subsurface processes such as plate tectonics or volcanism, and is thought to have evolvedpredominantly under the influence of impacts.[13] Prominent surface features include multi-ring structures,variously shaped impact craters, and chains of craters (catenae) and associated scarps, ridges anddeposits.[13] At a small scale, the surface is varied and consists of small, bright frost deposits at the tops ofelevations, surrounded by a low-lying, smooth blanket of dark material.[4] This is thought to result fromthe sublimation-driven degradation of small landforms, which is supported by the general deficit of smallimpact craters and the presence of numerous small knobs, considered to be their remnants.[14] Theabsolute ages of the landforms are not known.

Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide[6] and

probably molecular oxygen,[7] as well as by a rather intense ionosphere.[15]Callisto is thought to haveformed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation.[16] Its slowness and the lack of tidalheating prevented rapid differentiation. The slow convection in theinterior of Callisto, which commenced soon after formation, led to partial differentiation and possibly to theformation of a subsurface ocean at a depth of 100–150 km and a small, rocky core.[17]

The likely presence of an ocean within Callisto leaves open the possibility that it could harbor life.However, conditions are thought to be less favorable than on nearbyEuropa.[18] Various space probesfrom Pioneers 10 and 11 to Galileo and Cassini have studied the moon. Because of its low radiationlevels, Callisto has long been considered the most suitable place for a human base for future explorationof the Jovian system.[19]

Contents [hide]

• 1 Discovery and naming

• 2 Orbit and rotation

• 3 Physical characteristics

○ 3.1 Composition

○ 3.2 Internal structure

○ 3.3 Surface features

○ 3.4 Atmosphere and

ionosphere

• 4 Origin and evolution

• 5 Possibility of life in the

ocean

• 6 Exploration

• 7 Potential colonization

• 8 See also

• 9 Notes

• 10 References

• 11 External links

[edit]Discovery and naming

Callisto was discovered by Galileo in January 1610 along with three other large Jovian moons—Ganymede, Io, and Europa.[1] Callisto is named after one of Zeus's many lovers in Greekmythology. Callisto was a nymph (or, according to some sources, the daughter of Lycaon) who wasassociated with the goddess of the hunt, Artemis.[20]The name was suggested by Simon Marius soon afterthe moon's discovery.[21] Marius attributed the suggestion to Johannes Kepler.[20] However, the names oftheGalilean satellites fell into disfavor for a considerable time, and were not revived in common use untilthe mid-20th century. In much of the earlier astronomical literature, Callisto is referred to by its Romannumeral designation, a system introduced by Galileo, as Jupiter IV or as "the fourth satellite of Jupiter".[22] In scientific writing, the adjectival form of the name is Callistoan,[23] pronounced /ˌkælɨˈstoʊən/,or Callistan.[14]

[edit]Orbit and rotation

Callisto (bottom left), Jupiter (top right) and Europa (below and left of Jupiter'sGreat Red Spot) as viewed by Cassini

Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of approximately1 880 000 km (26.3 times the 71 398 km radius of Jupiter itself).[2] This is significantly larger than theorbital radius—1 070 000 km—of the next-closest Galilean satellite, Ganymede. As a result of thisrelatively distant orbit, Callisto does not participate in the mean-motion resonance—in which the threeinner Galilean satellites are locked—and probably never has.[9]

Like most other regular planetary moons, Callisto's rotation is locked to be synchronous with its orbit.[3] The length of the Callistoan day, simultaneously its orbital period, is about 16.7 Earth days. Its orbit isvery slightly eccentric and inclined to the Jovian equator, withthe eccentricity and inclination changing quasi-periodically due to solar and planetary gravitationalperturbations on a timescale of centuries. The ranges of change are 0.0072–0.0076 and 0.20–0.60°,respectively.[9] These orbital variations cause the axial tilt (the angle between rotational and orbital axes)to vary between 0.4 and 1.6°.[24]The dynamical isolation of Callisto means that it has never been appreciably tidally heated, which has hadimportant consequences for its internal structure and evolution.[25]Its distance from Jupiter also means thatthe charged-particle flux from the planet's magnetosphere at its surface is relatively low—about 300 timeslower than, for example, that at Europa. Hence, unlike the other Galilean moons, charged-particle irradiation has had a relatively minor effect on the Callistoan surface.[10] The radiation level at thesurface of Callisto is equivalent to a dose of about 0.01 rem (0.1 mSv) per day.[26]

[edit]Physical characteristics[edit]Composition

Near-infrared spectrum of a dark cratered plains area, indicating the presence of relatively less water (between 1 and2micrometers) and more rocky material than in impact basins

The average density of Callisto, 1.83 g/cm3,[3] suggests a composition of approximately equal parts ofrocky material and water ice, with some additional volatile ices such as ammonia.[11] The mass fraction ofices is between 49–55%.[11][17] The exact composition of Callisto's rock component is not known, but isprobably close to the composition of L/LL type ordinary chondrites, which are characterized by lesstotal iron, less metallic iron and more iron oxide than H chondrites. The weight ratio of iron tosilicon is0.9:1.3 in Callisto, whereas the solar ratio is around 1:8.[11]

Callisto's surface has an albedo of about 20%.[4] Its surface composition is thought to be broadly similar toits composition as a whole. Near-infrared spectroscopy has revealed the presence of water ice absorptionbands at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers.[4] Water ice seems to be ubiquitous onthe surface of Callisto, with a mass fraction of 25–50%.[12] The analysis of high-resolution, near-infrared and UV spectra obtained by the Galileo spacecraft and from the ground has revealed variousnon-ice materials: magnesium- and iron-bearing hydrated silicates,[4] carbon dioxide,[27] sulfur dioxide,[28] and possibly ammonia and various organic compounds.[4][12] Spectral data indicate that the moon'ssurface is extremely heterogeneous at the small scale. Small, bright patches of pure water ice areintermixed with patches of a rock–ice mixture and extended dark areas made of a non-ice material.[4][13]The Callistoan surface is asymmetric: the leading hemisphere—the hemisphere facing the direction of theorbital motion[g]—is darker than the trailing one. This is different from other Galilean satellites, where thereverse is true.[4] The trailing hemisphere[g] of Callisto appears to be enriched in carbon dioxide, while theleading hemisphere has more sulfur dioxide.[29] Many fresh impact craters like Lofn also show enrichmentin carbon dioxide.[29] Overall, the chemical composition of the surface, especially in the dark areas, maybe close to that seen on D-type asteroids,[13] whose surfaces are made of carbonaceous material.[edit]Internal structure

Model of Callisto's internal structure showing a surface ice layer, a possible liquid water layer, and an ice-rock interior

Callisto's battered surface lies on top of a cold, stiff, and icy lithosphere that is between 80 and 150 kmthick.[11][17] A salty ocean 50–200 km deep may lie beneath the crust,[11][17]indicated by studies ofthe magnetic fields around Jupiter and its moons.[30][31] It was found that Callisto responds to Jupiter'svarying background magnetic field like a perfectlyconducting sphere; that is, the field cannot penetrateinside the moon, suggesting a layer of highly conductive fluid within it with a thickness of at least 10 km.[31] The existence of an ocean is more likely if water contains a small amount of ammonia orother antifreeze, up to 5% by weight.[17] In this case the ocean can be as thick as 250–300 km.[11] Failingan ocean, the icy lithosphere may be somewhat thicker, up to about 300 km.

Beneath the lithosphere and putative ocean, Callisto's interior appears to be neither entirely uniform norparticularly variable. Galileo orbiter data[3] (especially the dimensionless moment of inertia[h]—0.3549 ± 0.0042—determined during close flybys) suggest that its interior is composed ofcompressed rocks and ices, with the amount of rock increasing with depth due to partial settling of itsconstituents.[11][32] In other words, Callisto is only partially differentiated. The density and moment of inertiaare compatible with the existence of a small silicate core in the center of the satellite. The radius of anysuch core cannot exceed 600 km, and the density may lie between 3.1 and 3.6 g/cm3.[3][11] Callisto'sinterior is in stark contrast to that of Ganymede, which appears to be fully differentiated.[12][33][edit]Surface featuresSee also: List of geological features on CallistoGalileo image of cratered plains, illustrating the pervasive local smoothing of Callisto's surface

The ancient surface of Callisto is one of the most heavily cratered in the solar system.[34] In fact,the crater density is close to saturation: any new crater will tend to erase an older one. The large-scale geology is relatively simple; there are no large Callistoan mountains, volcanoes or otherendogenic tectonic features.[35] The impact craters and multi-ring structures—together withassociated fractures, scarps and deposits—are the only large features to be found on the surface.[13][35]

Callisto's surface can be divided into several geologically different parts: cratered plains, light plains,bright and dark smooth plains, and various units associated with particular multi-ring structures andimpact craters.[13][35] The cratered plains constitute most of the surface area and represent the ancientlithosphere, a mixture of ice and rocky material. The light plains include bright impact craterslike Burr and Lofn, as well as the effaced remnants of old large craters called palimpsests,[i] the centralparts of multi-ring structures, and isolated patches in the cratered plains.[13] These light plains are thoughtto be icy impact deposits. The bright, smooth plains constitute a small fraction of the Callistoan surfaceand are found in the ridge and trough zones of the Valhalla and Asgard formations and as isolated spotsin the cratered plains. They were believed to be connected with endogenic activity, but the high-resolution Galileo images showed that the bright, smooth plains correlate with heavily fractured andknobby terrain and do not show any signs of resurfacing.[13] The Galileo images also revealed small, dark,smooth areas with overall coverage less than 10,000 km2, which appear to embay[j] the surroundingterrain. They are possible cryovolcanic deposits.[13] Both the light and the various smooth plains aresomewhat younger and less cratered than the background cratered plains.[13][36]Impact crater Hár with a central dome. Chainsof secondary craters from formation of the more recent crater Tindr at upperright crosscut the terrain.

Impact crater diameters seen range from 0.1 km—a limit defined by the imaging resolution—to over100 km, not counting the multi-ring structures.[13] Small craters, with diameters less than 5 km, havesimple bowl or flat-floored shapes. Those 5–40 km across usually have a central peak. Larger impactfeatures, with diameters in the range 25–100 km, have central pits instead of peaks, such as Tindr crater.[13] The largest craters with diameters over 60 km can have central domes, which are thought to resultfrom central tectonic uplift after an impact;[13] examples include Doh and Hár craters. A small number ofvery large—more 100 km in diameter—and bright impact craters show anomalous dome geometry. Theseare unusually shallow and may be a transitional landform to the multi-ring structures, as withthe Lofn impact feature.[13] Callistoan craters are generally shallower than those on the Moon.

The largest impact features on the Callistoan surface are multi-ring basins.[13][35] Two areenormous. Valhalla is the largest, with a bright central region 600 kilometers in diameter, and ringsextending as far as 1,800 kilometers from the center (see figure).[37] The second largest is Asgard,measuring about 1,600 kilometers in diameter.[37] Multi-ring structures probably originated as a result of apost-impact concentric fracturing of the lithosphere lying on a layer of soft or liquid material, possibly anocean.[23] The catenae—for example Gomul Catena—are long chains of impact craters lined up in straightlines across the surface. They were probably created by objects that were tidally disrupted as theypassed close to Jupiter prior to the impact on Callisto, or by very oblique impacts.[13] A historical exampleof a disruption was Comet Shoemaker-Levy 9.

As mentioned above, small patches of pure water ice with an albedo as high as 80% are found on thesurface of Callisto, surrounded by much darker material.[4] High-resolution Galileo images showed thebright patches to be predominately located on elevated surface features: crater rims, scarps, ridges andknobs.[4] They are likely to be thin water frost deposits. Dark material usually lies in the lowlandssurrounding and mantling bright features and appears to be smooth. It often forms patches up to 5 kmacross within the crater floors and in the intercrater depressions.[4]

Two landslides 3–3.5 km long are visible on the right sides of the floors of the two large craters on the right.

On a sub-kilometer scale the surface of Callisto is more degraded than the surfaces of other icy Galileanmoons.[4] Typically there is a deficit of small impact craters with diameters less than 1 km as comparedwith, for instance, the dark plains onGanymede.[13] Instead of small craters, the almost ubiquitous surfacefeatures are small knobs and pits.[4] The knobs are thought to represent remnants of crater rims degradedby an as-yet uncertain process.[14] The most likely candidate process is the slow sublimation of ice, whichis enabled by a temperature of up to 165 K, reached at a subsolar point.[4] Such sublimation of water orother volatiles from the dirty ice that is the bedrock causes its decomposition. The non-ice remnantsform debrisavalanches descending from the slopes of the crater walls.[14] Such avalanches are oftenobserved near and inside impact craters and termed "debris aprons".[4][13][14] Sometimes crater walls arecut by sinuous valley-like incisions called "gullies", which resemble certain Martian surface features.[4] Inthe ice sublimation hypothesis, the low-lying dark material is interpreted as a blanket of primarily non-icedebris, which originated from the degraded rims of craters and has covered a predominantly icy bedrock.

The relative ages of the different surface units on Callisto can be determined from the density of impactcraters on them. The older the surface, the denser the crater population.[38] Absolute dating has not beencarried out, but based on theoretical considerations, the cratered plains are thought to be~4.5 billion years old, dating back almost to the formation of the solar system. The ages of multi-ringstructures and impact craters depend on chosen background cratering rates and are estimated bydifferent authors to vary between 1 and 4 billion years.[13][34][edit]Atmosphere and ionosphere

Induced magnetic field around Callisto

Callisto has a very tenuous atmosphere composed of carbon dioxide.[6] It was detected bythe Galileo Near Infrared Mapping Spectrometer (NIMS) from its absorption feature near the wavelength4.2 micrometers. The surface pressure is estimated to be 7.5 × 10−12 bar (0.75 µPa) and particle density4 × 108 cm−3. Because such a thin atmosphere would be lost in only about 4 days (see atmosphericescape), it must be constantly being replenished, possibly by slow sublimation of carbon dioxide ice fromthe satellite's icy crust,[6] which would be compatible with the sublimation–degradation hypothesis for theformation of the surface knobs.

Callisto's ionosphere was first detected during Galileo flybys;[15] its high electron density of 7–17 × 104 cm−3 cannot be explained by the photoionization of the atmospheric carbon dioxide alone.Hence, it is suspected that the atmosphere of Callisto is actually dominated by molecular oxygen (inamounts 10–100 times greater than CO2).[7] However, oxygen has not yet been directly detected in theatmosphere of Callisto. Observations with the Hubble Space Telescope (HST) placed an upper limit on itspossible concentration in the atmosphere, based on lack of detection, which is still compatible with theionospheric measurements.[39] At the same time HST was able to detect condensed oxygen trapped onthe surface of Callisto.[40]

[edit]Origin and evolution

The partial differentiation of Callisto (inferred e.g. from moment of inertia measurements) means that ithas never been heated enough to melt its ice component.[17] Therefore, the most favorable model of itsformation is a slow accretion in the low-density Jovian subnebula—a disk of the gas and dust that existedaround Jupiter after its formation.[16] Such a prolonged accretion stage would allow cooling to largely keepup with the heat accumulation caused by impacts, radioactive decay and contraction, thereby preventingmelting and fast differentiation.[16] The allowable timescale of formation of Callisto lies then in the range0.1 million–10 million years.[16]

The further evolution of Callisto after accretion was determined by the balance of the radioactive heating,cooling through thermal conduction near the surface, and solid state or subsolidus convection in theinterior.[25] Details of the subsolidus convection in the ice is the main source of uncertainty in the models ofall icy moons. It is known to develop when the temperature is sufficiently close to the melting point, due tothe temperature dependence of ice viscosity.[41] Subsolidus convection in icy bodies is a slow process withice motions of the order of 1 centimeter per year, but is, in fact, a very effective cooling mechanism onlong timescales.[41] It is thought to proceed in the so-called stagnant lid regime, where a stiff, cold outerlayer of the moon conducts heat without convection, while the ice beneath it convects in the subsolidusregime.[17][41] For Callisto, the outer conductive layer corresponds to the cold and rigid lithosphere with athickness of about 100 km. Its presence would explain the lack of any signs of the endogenic activity onthe Callistoan surface.[41][42] The convection in the interior parts of Callisto may be layered, because underthe high pressures found there, water ice exists in different crystalline phases beginning from the ice I onthe surface to ice VII in the center.[25] The early onset of subsolidus convection in the Callistoan interiorcould have prevented large-scale ice melting and any resulting differentiation that would have otherwiseformed a large rocky core and icy mantle. Due to the convection process, however, very slow and partialseparation and differentiation of rocks and ices inside Callisto has been proceeding on timescales ofbillions of years and may be continuing to this day.[42]

The current understanding of the evolution of Callisto allows for the existence of a layer or "ocean" ofliquid water in its interior. This is connected with the anomalous behavior of ice I phase's meltingtemperature, which decreases with pressure, achieving temperatures as low as 251 K at 2,070 bar(207 MPa).[17] In all realistic models of Callisto the temperature in the layer between 100 and 200 km indepth is very close to, or exceeds slightly, this anomalous melting temperature.[25][41][42] The presence ofeven small amounts of ammonia—about 1–2% by weight—almost guarantees the liquid's existencebecause ammonia would lower the melting temperature even further.[17]

While Callisto is very similar in bulk properties to Ganymede, it apparently had a much simpler geologicalhistory. The surface appears to have been shaped mainly by impacts and other exogenic forces.[13] Unlikeneighboring Ganymede with its grooved terrain, there is little evidence of tectonic activity.[12] Explanationsthat have been proposed for the contrasts in internal heating and consequent differentiation and geologicactivity between Callisto and Ganymede include differences in formation conditions,[43] the greater tidalheating experienced by Ganymede,[44] and the more energetic impacts that would have been suffered byGanymede during the Late Heavy Bombardment.[45][46][47] The relatively simple geological history of Callistoprovides planetary scientists with a reference point for comparison with other more active and complexworlds.[12]

[edit]Possibility of life in the ocean

As with Europa and Ganymede, the idea has been raised that extraterrestrial microbial life may exist in asalty ocean under the Callistoan surface.[18] However, the conditions for life appear to be less favourableon Callisto than on Europa. The principal reasons are the lack of contact with rocky material and the lowerheat flux from the interior of Callisto.[18] Scientist Torrence Johnson said the following about comparing theodds of life on Callisto with the odds on other Galilean moons:[48]

Based on the considerations mentioned above and on other scientific observations, it is thought that of allof Jupiter's Galilean moons, Europa has the greatest chance of supporting microbial life.[18][49]

[edit]ExplorationThe Pioneer 10 and Pioneer 11 Jupiter encounters in the early 1970s contributed little new informationabout Callisto in comparison with what was already known from Earth-based observations.[4] The realbreakthrough happened later with the Voyager 1 and 2 flybys in 1979–1980. They imaged more than halfof the Callistoan surface with a resolution of 1–2 km, and precisely measured its temperature, mass andshape.[4] A second round of exploration lasted from 1994 to 2003, when the Galileo spacecraft had eightclose encounters with Callisto, the last flyby during the C30 orbit in 2001 came as close as 138 km to thesurface. The Galileo orbiter completed the global imaging of the surface and delivered a number ofpictures with a resolution as high as 15 meters of selected areas of Callisto.[13] In 2000,the Cassini spacecraft en route to Saturn acquired high-quality infrared spectra of the Galilean satellitesincluding Callisto.[27] In February–March 2007, the New Horizons probe on its way to Pluto obtained newimages and spectra of Callisto.[50]

Proposed for a launch in 2020, the Europa Jupiter System Mission (EJSM) is a joint NASA/ESA proposalfor exploration of Jupiter's moons. In February 2009 it was announced that ESA/NASA had given thismission priority ahead of the Titan Saturn System Mission.[51] ESA's contribution will still face fundingcompetition from other ESA projects.[52] EJSM consists of the NASA-led Jupiter Europa Orbiter, the ESA-led Jupiter Ganymede Orbiter, and possibly a JAXA-led Jupiter Magnetospheric Orbiter.

It was proposed that it could be possible to build a surface base on Callisto that would produce fuel forfurther exploration of the Solar System.[53] Advantages of a base on this moon include the low radiation(due to Callisto's distance from Jupiter) and geological stability. It could facilitate remote explorationof Europa, or be an ideal location for a Jovian system waystation servicing spacecraft heading farther intothe outer Solar System, using a gravity assist from a close flyby of Jupiter after departing Callisto.[19]In a December 2003 report, NASA expressed belief that an attempt for a manned mission to Callisto maybe possible in the 2040s.[55]